Remanent image erasure for rapid successive exposures

ABSTRACT

In apparatus employing an image orthicon, when a signal corresponding to the image focused on the orthicon has been obtained by scanning the target of the orthicon, the photocathode is evenly flooded with light and at the same time the relative photocathode, accelerator and target potentials are such that photocathode electrons arrive at the target at an energy level at which they do not cause secondary emission but are absorbed by parts of the target on which positive charge remains. This removes any remanent image, permitting the image orthicon to be used for different images in rapid succession.

United States atet [111 3,634,619

[72] Inventor Edward Thomas Astley 2,992,358 7/1961 Farnsworth Q 315/1 1 Potters Bars, England 3,433,995 3/1969 Scott 315/1 1 [21] P 843388 Primary Examiner-Robert L. Griffin [22] Filed July 22, 1969 Assistant ExammerGeorge G. Stellar [45] Patented 1972 AttorneyKemon Palmer & Estabrook [73] Assignee Crosfield Electronics Limited London, England ABSTRACT: In apparatus employing an image orthicon, when a signal corresponding to the image focused on the [54] FOR RAPID orthicon has been obtained by scanning the target of the Cl 2 D F orthicon, the photocathode is evenly flooded with light and at aims m I g the same time the relative photocathode, accelerator and tar- [52] US. Cl 178/7.2, get potentials are such that photocathode electrons arrive at l78/DIG. l, 3 l5/l l the target at an energy level at which they do not cause secon- [51] Int. Cl HOlj 31/48 dary emission but are absorbed by parts of the target on which [50] Field of Search l78/7.2, positive charge remains. This removes any remanent image, DIG. 1; 313/65; 3l5/l0, l1 permitting the image orthicon to be used for different images in ra id succession. [56] References Cited UNITED STATES PATENTS 2,853,648 9/1958 Theile 313/65 l A fmm 24 Zfla'M/m I m 9 2; me 1m? 9 J4. Z5 few/02b 5 F fiafcf/o g Ci IT H L T Ala/1a LF n 2) W I L 6 6700/? j" A? l\ J0 dd 4} n ,1 F M 2mm IV 46 I T "10/70 TL 4M9? FL d'm/e J z J4 40 /8 l 1] FE.

SHEET 1 OF 2 A tlorneys In venlo fiawzd K 4 PATEHTED JAN! 1 I972 PATENIEU JAN] I I972 SHEET 2 OF 2 REMANENT IMAGE ERASURE FOR RAPID SUCCESSIVE EXPOSURES When an image is focused on the photocathode of an image orthicon, the elemental areas of the photocathode emit electrons which vary in number in accordance with the light intensity of the image at that point of the photocathode, so that an electron image is produced by the photoemitting cathode surface. The electrons are accelerated towards a target by means of an accelerator electrode and reach the target with sufficient energy to cause secondary emission to take place. As a consequence, a replica of the original image is stored upon the target in the form of voltage charges, the more positive parts of the target representing the brightest parts of the image. To obtain an electric signal representative of the image, the target is scanned by a low-velocity electron beam. The more positive areas of the target abstract more electrons from the beam than the less positive areas, which correspond to the shadows in the image. The remaining part of the beam current returns to an electron multiplier to form the signal output.

As the target absorbs electrons from the beam, it cancels the positive charge at that point on the target and in theory complete cancellation of the stored image can be achieved. In practice, however, there is nearly always a small amount of remanent image on the target. This is normally ignored because the contents of the input scene change fairly slowly with respect to the frame scan rate.

There are, however, circumstances in which the remanent image is highly undesirable. If the input scene changes rapidly from one frame to the next, an object moving rapidly across the screen will appear to have a tail behind it, the tail being the remanent charge from a previous stored image. In some cases, the input scene may change completely.

According to the present invention, apparatus for operating an image orthicon includes means operative after the scanning of the target to flood the photocathode substantially evenly with light and means effective when the photocathode is flooded with light to hold the relative photocathode, accelerator and target potentials at values such that the photocathode electrons arrive at the target at an energy level at which they do not cause secondary emission but are absorbed by those parts of the target on which any positive charge remains. This solves the problem of the remanent image; the original potentials are restored in readiness for the next scene and frame scan. When scenes are flashed on to the photocathode, so that the stored image on the target is created instantaneously, the photocathode and accelerator potentials can be changed while scanning is proceeding so that they are ready for the target-clearing operation when the photocathode is uniformly flooded with light at the end of the frame forward scan. The potentials may then be restored during the frame flyback in readiness for the next image.

The use of an image orthicon in this way permits it to be used for flash photography work, for example.

In order that the invention may be better understood, one example of a circuit embodying the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 represents the circuit diagram in block form; and

FIG. 2 is a diagram showing waveforms at different points in the circuit of FIG. 1.

The circuit shown is for apparatus in which images are flashed on to an image orthicon. An object 4 to be exposed is located between a flash tube 6 and an image orthicon 8. When the flash tube is energized an image is formed on the photocathode 10 of the image orthicon, forming an electron image at its photoemitting surface. The electrons of this image are accelerated towards the target 12 by means of the accelerator electrode 14. As explained above, in an image orthicon secondary emission takes place from the target to cause a replica of the original image to be stored in the fonn of voltage charges. To enable a signal to be derived from the tube, the target is scanned by means of an electron gun 16, the electron beam from which discharges the target, the number of electrons abstracted from the beam at any point depending upon the voltage charge at that point of the target. The remaining beam electrons return to an electron multiplier 18 and form the signal output.

A triggering signal to cause the image to be flashed on to the photocathode is applied to the circuit on conductor 20. The triggering signal is shown at A in FIG. 2. An image flash circuit 22 receives this signal and applies an energizing signal to the flash tube 6. At the same time it generates a signal B (see also FIG. 2) to set a bistable circuit 24. The resulting signal changes at the outputs of the bistable circuit are shown by the leading edges of waveforms C and D. Waveform C starts a ramp generator 26 which applies a ramp waveform E to a frame deflection amplifier 23 supplying the frame deflection coils of the image orthicon. A line ramp generator 25 similarly supplies a ramp signal to a line deflection amplifier 27 which energizes the line deflection coils of the tube. In the example shown the line ramp generator is not synchronized with the frame deflection waveform. This is normally satisfactory and indeed preferable. In some cases, where the frame scan consists of only a few lines (for example, when scanning an alphanumerical character of small height) the first line of the scan may be a substantial part of the total scan height and it may then be desirable to synchronize the start of the frame ramp by means of a pulse from the line ramp generator.

The frame ramp waveform also goes to a voltage sensing circuit 28 which generates an output signal P when the ramp waveform reaches a predetermined level V (FIG. 2). The waveform F resets the bistable circuit 1, thereby tenninating the ramp output of the generator 26.

The output waveforms C and D of the bistable circuit 24 are also applied respectively to differentiating circuits 30 and 32 which produce output waveforms G and H, each of which contains voltage spikes of opposite polarities corresponding to the leading and trailing edges of waveforms C and D. The positivegoing leading edge pulse from the differentiating circuit 30 triggers a monostable circuit 34, which generates an output waveform I. The positive-going trailing edge pulse of waveform H triggers a monostable circuit 36 which generates an output waveform J. The wavefonns I and J are applied to differentiating circuits 38 and 40 respectively to derive waveforms K and L. Waveform K is applied directly to a second bistable circuit 42 and its positive-going voltage spike sets this circuit. The waveform L is applied through an OR- gate 44 to the other side of the bistable circuit 42 and the positive-going spike M of the waveform L resets the bistable circuit. The output of the latter is the waveform N.

It will be seen by reference to FIG. 2 that there is an interval, equal to the period of the monostable circuit 34, between the start of the ramp waveform and the start of the waveform N. Similarly, there is an interval, determined by the period of the monostable circuit 36, between the end of the ramp waveform and the trailing edge of the waveform N. This waveform is applied to amplifiers 46 and 48 which control the potentials applied to the photocathode and accelerator electrodes of the image orthicon. Thus, after the image of the object 4 has been flashed on to the photocathode and the image has been stored on the target 12, the photocathode and accelerator potentials are changed in relation to the target potential in readiness for the flooding operation at the end of the frame scan.

Flooding is controlled by a signal P from the monostable circuit 36. The waveform P is the inverse of waveform J and commences at the end of the frame ramp. It is applied to a clearing flash circuit 50 which then energizes a flash tube 52 the light from which exposes the photocathode directly. The photocathode is thus flooded with light at the end of the frame scan at a time when the relative photocathode, accelerator and target potentials are such that electrons emitted from the photocathode as a consequence of exposure to flash tube 52 arrive at the target at an energy level at which they do not cause secondary emission. They are absorbed by those parts of the target on which any positive charge remains. At the end of waveform N the photocathode and accelerator potentials return to their original values in readiness for the exposure of another image on to the photocathode 10.

In the circuit shown, it is necessary to ensure that the bistable circuit 42 is in the correct stage before the first image is flashed on to the image orthicon. This is achieved by applying clock pulses R to an AND-gate 543. The latter generates a waveform S containing a clock pulse only when the negativegoing signal D is not present. The pulse S goes to the OR-gate 44 to provide a resetting signal in the waveform M for the bistable circuit 42. Thus, when the equipment is first switched on the first clock pulse will ensure that the circuit 42 is in its correct condition and during the flash exposures, if the circuit 42 fails to reset in response to the normal resetting pulse, the clock pulse will ensure that it is reset before the next exposure takes place.

I claim:

1. Apparatus for operating an image orthicon which includes a photocathode, an accelerator, a target, an electron gun and deflection means, the apparatus comprising: means for focusing an image on to the photocathode of the image orthicon, means for establishing relative photocathode, accelerator and target potentials to cause the image to be stored on the target of the image orthicon in the form of electric charges; line and frame deflection generators for applying to the deflection means signals to cause an electron beam from the gun to scan the target to derive an output signal representing the said image; means for flooding the photocathode substantially uniformly with light; timing means for rendering the cathode flooding means operative only after the completion of the scanning of the stored image on the target; and means operative under the control of the timing means after the commencement of the scanning of the stored image on the target and before the flooding of the photocathode with light to modify the relative potentials of the photocathode, accelerator and target so that photocathode electrons arriving at the target when flooding takes place will have an energy level such that they will not cause secondary emission and will be absorbed by those parts of the target on which positive charge remains.

2. Apparatus in accordance with claim 1, in which said lastmentioned means operates during scanning of the target to modify the potentials of the photocathode and accelerator relative to that of the target, said modified potentials continuing until flooding of the photocathode has taken place.

3. Apparatus in accordance with claim 1, including a triggering circuit for energizing a flash tube to flash an image on to the photocathode, and in which the frame deflection generator includes a ramp generator, means responsive to the triggering circuit and connected to the ramp generator to initiate the operation of the ramp generator in response to a signal from the triggering circuit, the apparatus further including a delay circuit responsive to the triggering circuit and operating at the end of a delay period to modify the relative photocathode, accelerator and target potentials.

4. Apparatus in accordance with claim 1, including: a circuit for triggering a flash tube to flash an image on to the photocathode; a bistable circuit responsive to the triggering circuit to switch from a first condition to a second condition upon operation of the triggering circuit; a ramp generator in the frame deflection generator responsive to the bistable circuit so that the frame-scanning ramp waveform is generated while the bistable circuit is in its second condition; a first delay circuit, means connected between the bistable circuit and the first delay circuit for initiating the operation of the first delay circuit when the bistable circuit switches from its first to its second condition, and means responsive to the termination of the delay period of the first delay circuit to change the relative photocathode, accelerator and target potentials; a second delay circuit, means connected between the bistable circuit and the second delay circuit for initiating the operation of the second delay circuit when the bistable circuit switches from its second condition back to its first condition and means res on sive to the termination of the delay period of the second elay circuit to actuate a further flash tube for flooding the photocathode with light.

5. Apparatus in accordance with claim 4, in which the bistable circuit is set by the said triggering circuit and is reset by a voltage-sensitive circuit actuated by the ramp voltage output of the ramp generator when the said ramp voltage reaches a predetermined value. 

1. Apparatus for operating an image orthicon which includes a photocathode, an accelerator, a target, an electron gun and deflection means, the apparatus comprising: means for focusing an image on to the photocathode of the image orthicon, means for establishing relative photocathode, accelerator and target potentials to cause the image to be stored on the target of the image orthicon in the form of electric charges; line and frame deflection generators for applying to the deflection means signals to cause an electron beam from the gun to scan the target to derive an output signal representing the said image; means for flooding the photocathode substantially uniformly with light; timing means for rendering the cathode flooding means operative only after the completion of the scanning of the stored image on the target; and means operative under the control of the timing means after the commencement of the scanning of the stored image on the target and before the flooding of the photocathode with light to modify the relative potentials of the photocathode, accelerator and target so that photocathode electrons arriving at the target when flooding takes place will have an energy level such that they will not cause secondary emission and will be absorbed by those parts of the target on which positive charge remains.
 2. Apparatus in accordance with claim 1, in which said Last-mentioned means operates during scanning of the target to modify the potentials of the photocathode and accelerator relative to that of the target, said modified potentials continuing until flooding of the photocathode has taken place.
 3. Apparatus in accordance with claim 1, including a triggering circuit for energizing a flash tube to flash an image on to the photocathode, and in which the frame deflection generator includes a ramp generator, means responsive to the triggering circuit and connected to the ramp generator to initiate the operation of the ramp generator in response to a signal from the triggering circuit, the apparatus further including a delay circuit responsive to the triggering circuit and operating at the end of a delay period to modify the relative photocathode, accelerator and target potentials.
 4. Apparatus in accordance with claim 1, including: a circuit for triggering a flash tube to flash an image on to the photocathode; a bistable circuit responsive to the triggering circuit to switch from a first condition to a second condition upon operation of the triggering circuit; a ramp generator in the frame deflection generator responsive to the bistable circuit so that the frame-scanning ramp waveform is generated while the bistable circuit is in its second condition; a first delay circuit, means connected between the bistable circuit and the first delay circuit for initiating the operation of the first delay circuit when the bistable circuit switches from its first to its second condition, and means responsive to the termination of the delay period of the first delay circuit to change the relative photocathode, accelerator and target potentials; a second delay circuit, means connected between the bistable circuit and the second delay circuit for initiating the operation of the second delay circuit when the bistable circuit switches from its second condition back to its first condition and means responsive to the termination of the delay period of the second delay circuit to actuate a further flash tube for flooding the photocathode with light.
 5. Apparatus in accordance with claim 4, in which the bistable circuit is set by the said triggering circuit and is reset by a voltage-sensitive circuit actuated by the ramp voltage output of the ramp generator when the said ramp voltage reaches a predetermined value. 